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N. S. Sangeetha Santanu Pakhira Qing-Ping Ding Lennard Krause Hyung-Cheol Lee Volodymyr Smetana Anja-Verena Mudring Bo Brummerstedt Iversen Yuji Furukawa David
C. Johnston 《Proceedings of the National Academy of Sciences of the United States of America》2021,118(44)
SrMn2P2 and CaMn2P2 are insulators that adopt the trigonal CaAl2Si2-type structure containing corrugated Mn honeycomb layers. Magnetic susceptibility χ and heat capacity versus temperature T data reveal a weak first-order antiferromagnetic (AFM) transition at the Néel temperature K for SrMn2P2 and a strong first-order AFM transition at K for CaMn2P2. Both compounds exhibit isotropic and nearly T-independent , suggesting magnetic structures in which nearest-neighbor moments are aligned at to each other. The 31P NMR measurements confirm the strong first-order transition in CaMn2P2 but show critical slowing down above for SrMn2P2, thus also evidencing second-order character. The 31P NMR measurements indicate that the AFM structure of CaMn2P2 is commensurate with the lattice whereas that of SrMn2P2 is incommensurate. These first-order AFM transitions are unique among the class of (Ca, Sr, Ba)Mn2 (P, As, Sb, Bi)2 compounds that otherwise exhibit second-order AFM transitions. This result challenges our understanding of the circumstances under which first-order AFM transitions occur.The Mn-based 122-type pnictides ( Ca, Sr, Ba; Pn = P, As, Sb, Bi) have received attention owing to their close stoichiometric 122-type relationship to high- iron pnictides. The undoped Mn pnictides are local-moment antiferromagnetic (AFM) insulators like the high- cuprate parent compounds (1–3). The compounds crystallize in the body-centered tetragonal structure as in (A = Ca, Sr, Ba, Eu), whereas the compounds crystallize in the trigonal -type structure (4). Recently, density-functional theory (DFT) calculations for the 122 pnictide family have suggested that the trigonal 122 transition-metal pnictides that have the structure might compose a new family of magnetically frustrated materials in which to study the potential superconducting mechanism (5, 6). It had previously been suggested on theoretical grounds that CaMn2Sb2 is a fully frustrated classical magnetic system arising from proximity to a tricritical point (7–9).The electrical resistivity ρ and heat capacity versus temperature T of single-crystal CaMn2P2 were reported in ref. 10. The compound is an insulator at T = 0 and undergoes a first-order transition of some type at 69.5 K. The Raman spectrum of CaMn2P2 at T = 10 K showed new peaks compared to the spectrum at 300 K, whereas the authors’ single-crystal X-ray diffraction measurements showed no difference in the crystal structure at 293 and 40 K. They suggested that the results of the two types of measurements could be reconciled if a superstructure formed below 69.5 K (10). The authors’ magnetic susceptibility measurements below 400 K revealed no evidence for a magnetic transition.Here we report the detailed properties of trigonal CaMn2P2 and SrMn2P2 (11) single crystals. We present the results of single-crystal X-ray diffraction (XRD), electrical resistivity ρ in the ab plane (hexagonal unit cell) versus temperature T, isothermal magnetization versus applied magnetic field M(H), magnetic susceptibility , heat capacity , and 31P NMR measurements. We find from , and NMR that CaMn2P2 exhibits a strong first-order AFM transition at K whereas SrMn2P2 shows a weak first-order transition at K but with critical slowing down on approaching from above as revealed from NMR, a characteristic feature of second-order transitions. Thus, remarkably, the AFM transition in SrMn2P2 has characteristics of both first- and second-order transitions. The data also reveal the presence of strong isotropic AFM spin fluctuations in the paramagnetic (PM) state above up to our maximum measurement temperatures of 900 and 350 K for SrMn2P2 and CaMn2P2, respectively. This behavior likely arises from spin fluctuations associated with the quasi–two-dimensional nature of the Mn spin layers (12) together with possible contributions from magnetic frustration. Our single-crystal XRD data at room temperature and high-resolution synchrotron XRD data at T = 20 K for SrMn2P2 and CaMn2P2 demonstrate conclusively that there is no structure change of either compound on cooling below their respective .Our studies of SrMn2P2 and CaMn2P2 thus identify the only known members of the class of materials with general formula AMn containing Mn spins S = 5/2 that exhibit first-order AFM transitions, where A = Ca, Sr, or Ba and the pnictogen P, As, Sb, or Bi. In particular, only second-order AFM transitions are found in (13), (13–15), (8, 9, 16–19), (16, 19), and (20). 相似文献
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Sub-10 nm β-NaGdF4:18% Yb3+,2% Er3+ nanoparticles were synthesized in ethylene glycol and various ionic liquids under microwave heating. The products were characterized by powder X-ray diffraction, electron microscopy, and upconversion (UC) luminescence spectroscopy. After Yb3+ excitation at 970 nm, Er3+ ions are excited by energy transfer upconversion and show the typical green and red emission bands. The UC luminescence intensity was optimized with respect to reactant concentrations, solvents, and reaction temperature and time. The strongest UC emission was achieved for sub-20 nm core–shell nanoparticles which were obtained in the ionic liquid diallyldimethylammonium bis(trifluoromethanesulfonyl)amide from a two-step synthesis without intermediate separation. Strictly anhydrous reaction conditions, a high fluoride/rare earth ion ratio, and a core–shell structure are important parameters to obtain highly luminescent nanoparticles. These conditions reduce non-radiative losses due to defects and high energy acceptor modes of surface ligands. A low power excitation of the core–shell particles by 70 mW at 970 nm results in an impressive UC emission intensity of 0.12% compared to the bulk sample.The microwave-assisted synthesis of β-NaGdF4:Er3+,Yb3+ in anhydrous ionic liquids yields efficient upconversion luminescence nanoparticles. A core–shell structure raises the nanoparticle emission intensity to 0.12% of the bulk material. 相似文献
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